We show that the large amplitude X-ray variability common
for GBHCs (and AGN) cannot be explained by magnetic flares
if they are incoherent. Therefore, we present a cellular
automata model (CAM), in which magnetic flux tubes appear
randomly on the surface of the disk (with the rate
proportional to the local energy generation in the disk),
but do not flare until a certain criterion is reached.
As is already known from Solar CAMs, under these conditions
the magnetic flux tubes are in a self-organized critical
state, such that an ignition of one flux tube might prompt
its quasi-stable neighbors to flare as well, and a large
scale outburst may develop.

We present our model light curves and power spectra. These
show that the flares may only produce variability
shorter than about 0.1 sec, and that longer time variations
(often seen in GBHCs) must be due to temporal variations of
either the accretion disk itself or the efficiency with
which the flares are produced. Further, we find that there
is a strong connection between RMS amplitude of variability
and the state of the magnetic field in the disk. This fact
allows us to test the earlier suggested explanation of the
state transitions in GBHCs as due to the change in the state
of the field. We show that if the field is indeed largely
uniform in radiation-dominated disks (soft states of GBHCs)
and is highly localized in the gas-dominated disks (hard
states), then the X-ray RMS variability is self-consistently
lower in the soft state than it is in the hard state, in
agreement with observations.

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